[XLA][AMDGPU] use AddrSpaceCast to access GlobalVariable on AMDGPU

In XLA kernels pointers are in default address space (0). On LLVM AMDGPU
backend, all GlobalVariable instances must be in global address space (1).
Therefore in an AddrSpaceCast is required for AMDGPU backend. To cope with
existing NVPTX backend, use ConstantExpr::getPointerBitCastOrAddrSpaceCast
in this commit.